Oncogenes have become the central concept in understanding cancer biology and may provide valuable targets for therapeutic drugs. One gene which has captured the attention of many scientists is the human proto-oncogene, c-Ha-ras. This gene acts as a central dispatcher, relaying chemical signals into cells and controlling cell division. Ras gene alteration may cause the gene to stay in the "on" position. The ras oncogene is believed to underlie up to 30% of cancer, including colon cancer, lung cancer, bladder and mammary carcinoma. Bos, J. L., Cancer Res. 49:4682-4689 (1989). The ras oncogene has therefore become a target for therapeutic drugs.
All oncogenes and their products operate inside the cell. This makes protein-based drugs ineffective since their specificity involves ligand-receptor recognition. However, antisense oligodeoxyribonucleotides (oligonucleotides) are capable of easily entering cells and they can specifically target oncogenes. Wickstrom, E. (ed). Prospects for antisense nucleic acid therapy of cancer and Aids. New York: Wiley-Liss, Inc. 1991; Murray, J. A. H. (ed). Antisense RNA and DNA New York: Wiley-Liss, Inc. 1992. Antisense drugs are modified synthetic oligonucleotides that work by interfering with ribosomal translation of the target mRNA. The antisense drugs developed thus far destroy the targeted mRNA by binding to it and triggering ribonuclease H (RNase H) degradation of mRNA. The antisense compounds in clinical trials are all approximately from 17 to 25 nucleotides long. Oligonucleotides have a half-life of about 20 minutes and they are therefore rapidly degraded in most cells. Fisher, T. L. et al., Nucleic Acids Res. 21:3857-3865 (1993). To increase the stability of oligonucleotides, they are often chemically modified, e.g., they are protected by a sulfur replacing one of the phosphate oxygens in the backbone (phosphorothioate). Milligan, J. F. et al., J. Med. Chem. 36:1923-1937 (1993); Wagner, R. W. et al., Science 260:1510-1513 (1993). However this modification can only slow the degradation of antisense and therefore large dosages of antisense drug are required to be effective. An additional problem with antisense drugs has been their nonspecific activities. Yet, despite these problems, many antisense drugs are in phase I, II and III of clinical trials.
There are several reports showing that oligonucleotides complementary to various sites of ras mRNA can inhibit synthesis of ras protein (p21) which decrease cell proliferation rate in cell culture. U.S. Pat. No. 5,576,208 (Monia et al.); U.S. Pat. No. 5,582,986 (Monia et al.); Daska, Y. et al., Oncogene Res. 5:267-275 (1990); Brown, D. et al., Oncogene Res. 4:243-252 (1989); Saison-Behmoaras, T. et al., EMBO J. 10:1111-1116 (1991). Oligonucleotides complementary to the 5' flanking region of the c-Ha-ras RNA transcript has shown to inhibit tumor growth in nude mice for up to 14 days. Gray, G. D. et al., Cancer Res. 53:577-580 (1993). It was recently reported that an antisense oligonucleotide directed to a point mutation (G&gt;C) in codon 12 of the c-Ha-ras mRNA inhibited cell proliferation as well as tumor growth in nude mice when it was injected subcutaneously. U.S. Pat. No. 5,576,208 (Monia et al.); U.S. Pat. No. 5,582,986 (Monia et al.); Schwab, G. et al., Proc. Natl. Acad. Sci. USA 91:10460-10464 (1994). At the American Society of Clinical Oncology meeting in May 1997, researchers reported that antisense drugs shrank ovarian tumors in small clinical trials. Roush, et al. Science 276:1192-1194 (1997). At the same meeting it was reported that an oligonucleotide which blocks replication of cytomegalovirus that destroys the retinas of many AIDs patients, is already in phase III clinical trials.
Despite the optimism surrounding these studies, there are a number of serious problems with the use of antisense drugs such as difficulty in getting a sufficient amount of antisense into the cell, non-sequence-specific effects, toxicity due to the large amount of sulfur containing phosphothioates oligonucleotides and their inability to get into their target cells, and high cost due to continuous delivery of large doses. It would thus be desirable to provide a composition which overcomes these difficulties.